President Biden, in a major shift pressed by his advisers and key allies, has authorized Ukraine to conduct limited strikes inside Russia with American-made weapons, opening what could well be a new chapter in the war for Ukraine, U.S. officials said on Thursday.

Mr. Biden’s decision appears to mark the first time that an American president has allowed limited military responses on artillery, missile bases and command centers inside the borders of a nuclear-armed adversary. White House officials insisted, however, that the authorization extended only to what they characterized as acts of self-defense, so that Ukraine could protect Kharkiv, its second-largest city, and the surrounding areas from missiles, glide bombs and artillery shells from just over the border.

“The president recently directed his team to ensure that Ukraine is able to use U.S.-supplied weapons for counter-fire purposes in the Kharkiv region so Ukraine can hit back against Russian forces that are attacking them or preparing to attack them,” a U.S. official said in a statement issued by the administration. “Our policy with respect to prohibiting the use of ATACMS or long-range strikes inside of Russia has not changed,” the statement continued, referring to an artillery system, provided to Ukraine, that has the capability to reach deep inside Russian territory.

The decision by Mr. Biden was reported earlier on Thursday by Politico.

American officials said that the change in policy went into effect on Thursday.

Though the White House cast the decision as a narrow one, allowing the Ukrainians to strike pre-emptively if they see evidence of preparations for an attack, or in response to a Russian barrage near Kharkiv, the implications are clearly much broader. Until now, Mr. Biden has flatly refused to let Ukraine use American-made weapons outside of Ukrainian borders, no matter what the provocation, saying that any attack on Russian territory risked violating his mandate to “avoid World War III.”

But having reversed his position, even in limited circumstances, Mr. Biden has clearly crossed a red line that he himself drew. And administration officials conceded that if Russia mounted other attacks from inside its territory beyond Kharkiv, the president’s restrictions could be subject to further loosening. “This is a new reality,” one senior official said, declining to speak on the record, “and perhaps a new era” in the Ukraine conflict.

Much may depend on how the Russians react to the change in the next days and weeks — or whether they react at all. Russia has warned that it will respond, in unspecified ways, if the United States shifts policy. Last week, as word of an impending change took place, Russia conducted drills for the forces that move and deploy tactical nuclear weapons, in what appeared to be a signal to Washington.

Russia has repeatedly played the nuclear card in the 27 months since it invaded Ukraine, mostly notably in October 2022, when it appeared the entire Russian military invasion of Ukraine could collapse. Gen. Mark A. Milley, then the chairman of the Joint Chiefs of Staff, often talked about the “nuclear paradox,” that the closer the Russians came to losing in Ukraine, the higher the nuclear peril.

But now Mr. Biden’s reversal raises a new question: How will Russia react to strikes that employ American weapons inside its territory? It is impossible to know exactly where President Vladimir V. Putin of Russia will draw his red line. Mr. Putin has not responded to Britain’s decision to ease the restrictions on its weapons, but in the Russian leader’s mind, the United States is a different kind of rival.

Inside the White House, Mr. Biden’s deliberations were very closely held, known only to a very narrow group of aides. But The New York Times revealed last week that Secretary of State Antony J. Blinken had come back from a sobering trip to Kyiv and privately told the president that his 27-month-long ban against shooting American weapons into Russian territory was now placing parts of Ukraine in peril. The Russians, he said, were exploiting the president’s ban and mounting constant attacks from a safe haven just inside the Russian border.

But by that time, Jake Sullivan, the president’s national security adviser, and Defense Secretary Lloyd J. Austin III had already concluded that the geography of the battle around Kharkiv would require an exception to the hard rule that the United States had set against firing into Russia, senior officials said. Ukraine was suffering from what one official called “an artificial line” in the middle of the battlefield that kept them from responding to devastating attacks. Mr. Sullivan and Mr. Austin concluded that it made no sense to restrict the Ukrainians from responding — even while maintaining a ban on using American equipment for long-range strikes deep into Russia.

Some American allies had already gone further. Britain weeks ago allowed Ukraine to use its Storm Shadow long-range missile systems for attacks anywhere in Russia, and France and Germany recently took the same position. So did Jens Stoltenberg, the secretary general of NATO.

From the outside, it appeared that each of those countries was mounting a campaign to get Mr. Biden to change his mind. But American officials insisted that only Britain reached its decision before Washington did, and that by the time the major European allies supported the change, they had been told Mr. Biden was headed in the same direction.

The decision follows weeks of intense behind-the-scenes conversation with the Ukrainians, made more urgent after Russia began a major assault on Kharkiv around May 10.

Three days later, on May 13, Mr. Sullivan, Mr. Austin and the chairman of the Joint Chiefs of Staff, Gen. Charles Q. Brown Jr., held one of their regularly scheduled secure video conferences with their Ukrainian counterparts. Once again, the Ukrainians pressed for Mr. Biden to lift U.S. restrictions on firing into Russian territory, arguing that the president’s concerns about escalation were overblown. But now, they said, the issue had become more urgent because the Russians were shelling civilian sites around Kharkiv from inside their border — knowing that the Ukrainians could not fully respond.

After the meeting, officials said, Mr. Sullivan, Mr. Austin and General Brown decided to recommend to the president that he reverse his position. But they kept the decision very close. Two days later, on May 15, Mr. Sullivan conveyed the recommendation to Mr. Biden, who — for the first time — said he was inclined to carve out an exception that would allow the Ukrainians to strike back, even if Russian attacks were coming from just a few miles behind the Russian border. By then, Mr. Blinken was already in Kyiv and had heard the case for a reversal directly from President Volodymyr Zelensky.

The same day as the private meeting with Mr. Sullivan, the president saw Gen. Christopher G. Cavoli, the four-star commander of the U.S. European Command and the supreme allied commander for Europe. He was in Washington for an annual meeting of all the combatant commanders, and told Mr. Biden that he also agreed that the ban on firing into Russia was posing a danger to Ukraine — though he, too, one official said, was concerned about the possible Russian reactions.

Mr. Blinken returned from Kyiv and saw Mr. Biden and Mr. Sullivan on the evening of May 17 in the Oval Office, saying that he emerged convinced that the United States had to alter its stance. It was clear by then that Mr. Biden was in agreement, officials said, but the president insisted that before he issued a formal decision, he wanted a meeting of his national security “principals” to consider the risks. That meeting did not take place until last week, just as news of Mr. Blinken’s change of view leaked out.

White House officials were clearly angry about the leak, and some said they were worried it would tip off the Russians or interfere with the final decision-making. The formal orders did not get conveyed to the Pentagon until earlier this week. Mr. Blinken, who knew the change was coming, hinted at the possibility in Moldova, where he left open the possibility that the United States might “adapt and adjust” its stance because the situation on the ground had changed. But he did not say that the president had already reversed course, and White House officials refused to comment.

Mr. Biden has never publicly commented on the internal debate that led him to change his approach. So it is unclear whether he now believes that the risk of escalation — including nuclear escalation — has declined, or whether the prospect that Ukraine might lose more territory changed his view.

So few members of the National Security Council or the Pentagon knew of the change that a Pentagon spokeswoman, Sabrina Singh, was still defending the old policy on Thursday afternoon in a briefing for reporters. She repeatedly said that there was no change. “The security assistance that we provide Ukraine is to be used within Ukraine, and we don’t encourage attacks or enable attacks inside of Russia,” she said.

But she insisted that Ukraine could be effective by focusing on tactical and operational targets that directly influence the conflict within its boundaries, she said. “So our policy hasn’t changed.”

In fact, it had, days before. No one had told her, defense officials say, that Mr. Austin had already released orders to allow Ukraine to open fire, with American weapons, on military targets over the Russian border. U.S. officials now say they expect that the first counterattacks with American weapons will begin within hours or days.